Since it was so miserably overcast/misty/foggy today I decided to do some chrono work.

I got some free 125-gr X-Treme CPHPs for .38/.357 awhile back and have been trying to work up a viable and SPEEDY load. HS-6 worked OK but not great. I have a partial jar of IMR PB laying around, though, so figured I'd give it a shot. There's plenty of reference (jacketed) data for everything from soft .38 Specials (3.6-4.1 grains) to moderate .357 Magnum loads (4.5-5.9 grains). Thus, I built a ladder of 3.7, 3.9 & 4.0 grains in .38 Special casings and 4.5, 4.8, 5.1, 5.4 & 5.7 grains in .357 Magnum casings. All were primed with CCI-500 SPP.

Meteorological Data:44 F, 91% RH, 30.15" Hg

The .38 Special loads barely left the barrel they were so soft. I was so concerned that I checked the barrel for obstructions after each shot. Finally got one bullet from each across the chrono, but I think they stopped for a cigarette halfway there first.

This just sucks. Even the highest load for .357 Magnum barely yielded .38 Special velocity. The whole point to running 125-gr slugs was to play with higher speeds, something I've now failed at with two different powders.

Obviously, I could try magnum small pistol primers. That MIGHT help a little bit. Back when I played with HS-6 I used WSP(m) primers though, still not very impressive.

I have a go forward plan. I've got some Accurate #5 kicking around that I'll try next. Maybe try a W-231 load, too. But in the meantime I am wondering WHAT DID I DO WRONG with the IMR PB to get these horrendous results? Is it possible the powder has just gone bad (second owner, jar opened before I got it with a lot of other reloading stuff)? Have others found IMR PB load data to be on the atrociously low side?

I did some other chrono work today that gave believable results (all cowboy loads with lead or coated bullets) so I'm not suspecting that my chrono is bad. Santa failed to come through on the new chrono, though.

You're confusing powders. While I previously tried HS6 in .357 Mag with this bullet (in loads exactly as shown on the Hodgdon site, through not getting anywhere close to the velocity) the powder charges reported in this thread are for IMR PB.

The linked Hodgdon data agrees with what I loaded (and gave awful slow results).

I use IMR PB for my 357. I load Extreme 158 gr SWC. I found and old IMR load publication on the internet. I tried the data from Hogdon and I was having the same problem your having. I don't have a crono but I would say they are 38 +p in 357 cases with a 158 gr swc